Full color printing has become a desired goal of office products. Two different types of full color printers which have significant potential for fulfilling such goals are the ink jet printer and the electrophotographic printer. Color printing is accomplished by providing multiple layers, or separations, of ink on a page. Commonly, colors are provided by subtractive combinations of cyan, magenta and yellow inks. To print black, a combination of equal amounts of cyan, magenta and yellow is printed, or a fourth black ink is used as a substitute. Undercolor removal, a well known process in the printing arts, can be used to print a single layer of black ink as a substitute for the combination of equal amounts of cyan, magenta and yellow. For a fuller discussion of under color removal and its application to electronically derived or created images, reference is made to J. A. C. Yule, Principles of Color Reproduction. (John Wiley & Sons, Inc., New York, 1967), pages 294-327.
Generally, printers can deposit colors in one of two processes: 1) all four colors can be deposited at about the same time in parallel processing arrangement (common, although not required or limited to color ink jet printers), or 2) all four colors can be deposited sequentially in a serial processing arrangement (common, although not required or limited to color electrophotographic printers). Combinations of these two methods are possible.
A problem of color printers generally is that when too much marking material is used, undesirable image artifacts and printing defects occur. In liquid marking material printers such over coverage is characterized by the problems of ink puddling or pooling, bleeding to adjacent image areas, and flow through to the back side of the receiving material. Paper cockle is also a problem due to saturation of the paper receiving material and subsequent rapid drying. In powdered marking material printers, paper curl and cockle is caused by differential shrinkage of toner and paper in the printing process. In both liquid marking and powder marking printers, coverage reduction as in U.S. patent application Ser. No. 07/917,643 by Klassen, filed Jul. 23, 1992, reduces marking material coverage in documents including heavily saturated regions of continuous tone images (see also, Klassen, "Reducing Ink Coverage Levels in Binary CMYK Images", Proc. Soc. Imaging Science and Technology, 46th Annual Conference (May, 1993), pp. 173-175). To prevent artifacts from occurring in the pixel reduction step, a processing path through each given area is used which tends to "randomize" the turn off effect. However, this process assumes the availability of binary print driver signals. For cost reasons, it is usually desirable for the process to operate irrespective of image content, or on the separation binary bitmaps without further image information.
In sequential separation printing machines, particularly characterized by electrophotographic printers such as the Xerox 5775 Digital Color Copier, while all four separations are available in the original continuous tone format (e.g., multibit signals, typically 8 bits per separation pixel), only the separation being currently marked is available in bitmap format, taking into account color correction and halftoning (for a more complete discussion of color correction, see, for example U.S. Pat. No. 5,305,119 to Rolleston et al.; U.S. Ser. No. 08/131,168, filed Oct. 4, 1993, entitled "Reduced Storage of Pre-Computed Difference Tables Used In Color Space Conversion", by R. J. Rolleston (assigned to the same assignee as the present application); U.S. Ser. No. 08/144,987, filed Oct. 29, 1993, entitled "Color Printer Calibration Test Pattern" by R. J. Rolleston et al. (assigned to the same assignee as the present application); U.S. Ser. No. 08/179,284, filed Jan. 10, 1994, entitled "Color Printer Calibration Architecture", by R. J. Rolleston et al. (assigned to the same assignee as the present application); U.S. Ser. No. 08/223,494, filed Apr. 5, 1994, entitled "Color Printer Calibration with Improved Color Mapping Linearity", by R. J. Rolleston (assigned to the same assignee as the present application); and U.S. Ser. No. 08/254,629, filed Jun. 6, 1994, entitled "Color Printer Calibration with Blended Lookup Tables", by R. J. Rolleston et al. (assigned to the same assignee as the present application). Since processes previously described for coverage reduction assume the presence of the entire image in a print ready format, they can base reduction actions on multiple separations. However, if a process has only a single separation in a print ready format, the reduction process cannot refer to other separations.
Tasaki and Shiga (U.S. Pat. No. 5,237,344) describe a method for reducing the amount of ink printed to 50%, 75% or 66%. The method uses fixed patterns of turn-off locations (e.g., a checkerboard for 50%) and selects the pattern based on the printing mode (reverse character mode, block graphic mode or normal character mode), the character selected, and possibly the relative humidity. Apparently, the method is designed for single color (black) printing: if it were used for multiple separation (e.g., red formed from yellow and magenta) printing, both separations would be turned off in the same place, resulting in more obvious patterns. The small set of fixed turn-off patterns makes the method very sensitive to line angle, as lines at some angles will have more pixels turned off than others. Also the method is only useful for characters from a built-in font, including graphic characters: arbitrary fonts and shapes, such as are requested in documents created using industry standard page description languages e.g. PCL or PostScript, cannot be handled in this way.
U.S. Pat. No. 4,930,018 to Chan et al. teaches reduction of paper cockle and graininess of ink jet prints. Printing of a given scan line occurs multiple times, with three different dye loadings, with pixels requiring the highest dye loading printed on one pass, pixels requiring an intermediate dye loading printed on another pass, and pixels requiring the lowest dye loading on another pass. The method takes as input continuous tone RGB (red--green--blue) images and performs RGB-CMYK (cyan--magenta--yellow key or black) conversion with full under color removal. As understood, printing is performed at half resolution, so that "pixels" in the input image correspond to 2.times.2 blocks in the output image. The image data is first error diffused from 8 bits per pixel per separation to 4 bits pixel per separation. Then, for each pixel, a count of up to 4 drops of each dye loading is computed, for each separation. There are multiple choices, ranked in order of total ink coverage. If the highest coverage choice exceeds the maximum allowable coverage, the separation with highest coverage is changed to use a lower coverage value for the same gray level, if possible. If it is not possible to stay at the same gray level, the gray level for that separation is dropped by one, and the error passed on to neighbors. The process iterates until the total ink coverage is as low as required. Pixels within the 2.times.2 block are assigned values (0 or 1) by proceeding around the block in clockwise order, and filling in pixels in order. First, the high dye load pixels are turned on, then the medium, then the low. Within each dye loading group, first black is turned on, until there are no more black pixels of that dye loading, then the next pixels in the cycle are cyan, until there are no more cyan required, then magenta, and yellow, and then the next dye load group. By maximizing ink coverage and using multiple dye loadings, they reduce the noisiness of the image, and by maintaining the total ink coverage within known limits, they prevent the many problems associated with excessive ink.
U.S. Pat. No. 4,999,646 to Trask teaches limiting coverage to 100% coverage (by the above definition of coverage), or perhaps between 100 and 200% coverage (if 100% corresponds exactly to no white spaces on a page), owing to the circular shape and overlap of print dots. Coverage is limited by using 2.times.2 super pixels and assigning each one drop per pixel in a combination that depends on the color required. Assuming one bit per separation input with full undercolor removal, there are eight possible colors that could be requested (including white). In order to reduce patterning due to the multiple swaths, two passes are used, each of a checkerboard pattern of pixels (the two passes being offset to provide full coverage). The two pass process allows ink to dry between passes.
The above-identified references are incorporated by reference for their teachings.